The present invention relates to a dynamic pressure bearing, and more particularly to a dynamic pressure bearing which is suitably used in a part where corrosion resistance is requested, such as a hard disk drive.
In recent years, dynamic pressure bearings are often used as bearings for an apparatus in which rotation at high speed with high accuracy is required. In a dynamic pressure bearing, generally, a working fluid such as lubricating oil is poured into a space between a shaft and a bearing, and fine dynamic pressure grooves are formed on either one of the shaft or the bearing. The pressure of the working fluid is raised by means of a pumping action or the like caused by the relative rotation of the shaft and the bearing. By means of the pressure, the shaft is supported in a relatively rotatable manner under a substantially non-contact condition with respect to the bearing (for example, see Patent literature 1).
A known suitable application of such a dynamic pressure bearing is a dynamic pressure bearing in a hard disk drive. The dynamic pressure bearing in this use usually adopts a structure in which dynamic pressure generating grooves of, for example, a herringbone pattern are formed in an inner peripheral face of a sleeve, and a shaft is passed through the inside of the sleeve. For such a dynamic pressure bearing to be used in a hard disk drive, facility in working, a sliding characteristic, wear resistance, and corrosion resistance are particularly required. In order to satisfy the requirements, a conventional dynamic pressure bearing for a hard disk drive adopts a structure in which a Cu-Zn copper alloy such as brass is used as a base material, and a coating layer of hard Ni plating is formed on the surface.
Patent literature 1. . . Japanese Patent Publication (Kokai) No. 11-223214
In accordance with speeding up and miniaturization, recently, a dynamic pressure bearing is requested to have high working accuracy such as size accuracy of the inner diameter (the clearance with respect to the shaft). In the case where Ni plating is applied to the surface as described above, therefore, there arises a problem in that the production yield is lowered because of dispersion in thickness of the plating layer.
In order to solve the problem, a coatingless structure has been studied. In the same manner as an existing copper alloy, conventionally, a copper alloy which is to be used in a dynamic pressure bearing contains a soft metal component having a low melting point which is easily separated and dispersed as a result of supersaturation, such as Pb or Bi in order to improve the free-cutting property. Accordingly, it has been pointed out that the low-melting metal which is separated and dispersed, such as Pb or Bi dissolves in the working fluid of the dynamic pressure bearing, whereby the working fluid is deteriorated to cause a lubrication failure. In addition, a copper alloy such as brass is not regarded as satisfying the requirements of the wear resistance and the corrosion resistance.